Demand for full color image forming and not just monochrome image forming has recently grown with respect to electrophotographic image forming apparatuses, and development of electrophotographic full color image forming apparatuses is proceeding. Ordinarily, full color image forming apparatuses form images by using toner of corresponding colors for image data of a plurality of colors in which a color image has undergone color separation. For example, a full color image is formed by reading a color image via filters for colors of the three additive primary colors (red, green, and blue), creating image data for at least the three subtractive primary colors (cyan, magenta, and yellow) from the read data, creating visible images using toner of corresponding colors based on the image data for each color, and overlaying the visible images for each color.
Such full color image forming apparatuses require exposure, developing, and image transfer steps for each color and face problems with aligning the visible images in each color, and are therefore viewed as having an image forming speed for full color images which is slower than an image forming speed for monochrome images.
Tandem-type full color image forming apparatuses have therefore been proposed in the past, in which a semi-conducting endless belt is rotatably provided, a plurality of image forming portions for individually forming visible images of mutually differing colors is disposed in a row along a moving direction of an outer circumferential surface of the endless belt, and one full color image is formed during at least one rotation of the endless belt.
With a tandem-type full color image forming apparatus, full color image formation is made faster by employing an intermediate image transferring method in which the visible images in each color, formed by the image forming portions, are transferred to paper after being overlaid on an outer circumferential surface of the endless belt, or an image transferring carrying method in which the visible images in each color, formed by the image forming portions, are sequentially transferred onto a surface of the recording medium which is carried by adhering to an outer circumferential surface of the endless belt. During full color image formation, an outer circumferential surface of the endless belt in the intermediate image transferring method and a surface of a recording medium which is carried by the endless belt in the image transferring carrying method abut at a predetermined pressure the image bearing members which are individually provided in the image forming portions.
The image transferring carrying method transfers visible images formed by the image forming portions directly to the surface of the recording medium, and therefore has the advantage of only having to perform the image transferring step once for each color, but since the recording medium is carried by the endless belt by being adhered to it, there is not only the disadvantage that a configuration is required for charging the recording medium, but also that the charge of the recording medium affects the transferring of the visible image, causing a deterioration in image formation. In contrast, the intermediate image transferring method has attracted attention because although the step of transferring the images is performed twice for each color, the image formation can be favorably done, as there is no need to charge the recording medium in order to carry it.
On the other hand, while it is true that demand for full color image forming has increased, black color monochrome image formation remains the mainstream, and even faster monochrome image formation is also in demand. Therefore, there is a need to resolve the slowness of monochrome image formation in full color image forming apparatuses due to the use of a plurality of image forming portions provided for full color image formation.
Moreover, during monochrome image formation in a tandem-type full color image forming apparatus, one image forming portion of a plurality of image forming portions forms images, while the remaining image forming portions form no images. There is also a need to prevent the deterioration of image bearing members and the mixing of toner due to contact between the image bearing members and the endless belt or recording medium in image forming portions which are not used for monochrome image formation.
The endless belt becomes deformed when the endless belt is abutted on the image bearing members during standby, when neither monochrome image formation nor full color image formation is performed. Further, any recording medium stuck between the image bearing members and the endless belt cannot be removed when jamming occurs, nor can part replacement or cleaning be performed during maintenance.
Accordingly, configurations have been disclosed which speed up monochrome image formation with conventional tandem-type full color image forming apparatuses, by providing an image forming portion for forming black images in addition to image forming portions for forming images in the three subtractive primary colors, and changing the position of the endless belt with respect to these image forming portions during monochrome image formation and during full color image formation (for example, Patent Documents 1 to 3).
With these conventional configurations, at least one of a driving roller and a driven roller, between which the endless belt is held in tension in order to be driven, is caused to move with respect to the image forming portions. With these configurations, the endless belt can be caused to move without having to give consideration to changes in the circumferential length of the endless belt, since the center distance between the driving roller and the driven roller, across which lies the endless belt in a tensioned condition, is maintained constant during movement of the endless belt.    Patent document 1: JP H10-039651A    Patent document 2: JP H10-293437A    Patent document 3: Japan Patent 2574804